The role of miRNAs (small non-coding RNAs) in regulation of gene expression is reported. By binding with target mRNAs miRNAs control expression of the genes encoding these mRNAs at post-transcriptional level taking part in physiological and pathological processes, from embryogenesis to neoplastic disorders. Various research teams have been studying the miRNA functions and mechanisms of action since the discovery of these molecules in 1993. The paper reports miRNA biogenesis pathways, modes of interaction between miRNAs and target mRNAs, and the mechanisms underlying suppression of translation and mRNA degradation. The results of numerous studies have shown that miRNAs can be used in medicine as biomarkers for diagnostic and prognostic purposes. Developments in miRNA therapeutics hold promise for the treatment of diseases, in which gene dysregulation plays a key role.